1. Field of the Invention
The present invention relates to a heat transfer pipe with grooved inner surface, more particularly, to a heat transfer pipe with grooved inner surface to be used for the heat exchange by evaporating or condensing for example refrigerant in the pipe.
2. Description of the Related Art
A heat transfer pipe has been used for a heat exchanger used in a refrigerating machine, an air conditioner, a heat pump, etc. In the heat transfer pipe, the heat exchange is conducted by evaporating or condensing the refrigerant provided therethrough.
An inner surface of a conventional heat transfer pipe was flat and smooth at first. However, as the investigation of thermodynamics advances, it is found that the heat transfer coefficient can be improved by forming a predetermined convexo-concave portion at the inner surface of the heat transfer pipe. Recently, the heat transfer pipe with grooved inner surface becomes the mainstream of the heat transfer pipe. The heat transfer pipe with grooved inner surface comprises a heat transfer pipe with an outer diameter of 5 to 9.52 mm, in which grooves with approximately trapezoidal cross section and fins for separating the grooves with approximately triangle cross sections are spirally formed at the inner surface. For example, page 138 of “Compact Heat Exchanger” by Hiroshi Seshimo discloses such a type of the heat transfer pipe with grooved inner surface.
FIGS. 1A to 1C are schematic illustrations showing a conventional heat transfer pipe for in-pipe evaporation/condensation (heat transfer pipe with grooved inner surface), wherein FIG. 1A is a cross sectional view of the heat transfer pipe including a pipe axis line (virtual axis), FIG. 1B is a cross sectional view of the heat transfer pipe cut along a line perpendicular to the pipe axis line, and FIG. 1C is an enlarged cross sectional view showing a part A shown in FIG. 1B. In FIGS. 1A to 1C, H is a fin height, β is an angle with respect to the pipe axis line (torsion angle), and W is a bottom width of the groove. A heat transfer pipe 1 with grooved inner surface comprises a pipe body 2 in which continuous spiral grooves 3 and spiral fins 4 are formed at an inner surface.
When such heat transfer pipe 1 with grooved inner surface is used, a surface area in the pipe becomes large, so that a heat transfer area can be increased. In addition, high evaporation heat transfer coefficient and condensation heat transfer coefficient can be provided by acceleration of turbulent flow effect and reduction in refrigerant liquid film thickness in accordance with the addition of the spiral fins. Therefore, performance of the refrigerating machine, air conditioning device, heat pump, etc. can be improved.
In late years, this kind of heat transfer pipe with grooved inner surface has been developed to have a groove shape with an improved vaporization property, by adding one or more fins having a fin height lower than the spiral fin positioned in a space between the spiral fins to keep the liquid film thin. For example, Japanese Patent Laid-Open No. 2002-350080 (JP-A-2002-350080) proposes such a heat transfer pipe with grooved inner surface. FIGS. 2A and 2B are schematic illustrations showing another conventional heat transfer pipe with grooved inner surface having low fins and high fins, wherein FIG. 2A is a cross sectional view of the heat transfer pipe cut along a line perpendicular to a pipe axis line, and FIG. 2B is an enlarged cross sectional view showing a part A shown in FIG. 2A.
In FIGS. 2A and 2B, a heat transfer pipe 10 with grooved inner surface comprises a pipe body 11, high fins 12a, and low fins 13a, and a whole structure is made of copper pipe. At an inner surface of the pipe body 11 (outer diameter of 7 mm, and a groove bottom wall thickness of 0.25 mm), the high fins 12a with a fin height of 0.2 mm and a torsion angle of 16° are formed. The number of the high fins is 50. At a groove bottom of a spiral groove 12b, two peaks of the low fins 13a with a fin height of 0.03 mm are formed between the adjacent high fins 12a. In FIG. 2B, HF is a fin height of the high fin 12a and hf is a fin height of low fin 13a. 
By using such heat transfer pipe 10 with grooved inner surface, a surface area increases more than the conventional heat transfer pipe with grooved inner surface, and thin liquid film can be formed by the presence of the low fin 13a, so that the vaporization property can be improved.
However, according to the heat transfer pipe with grooved inner surface proposed by JP-A-2002-350080, the fin height Hf of the high fin 12a is 0.2 mm and the fin height hf of the low fin 13a is 0.03 mm in the pipe body 11, so that a fin height ratio (the fin height hf of low fin/the fin height Hf of high fin) is 0.15. As shown FIG. 3, compared with the conventional heat transfer pipe with grooved inner surface (i.e. heat transfer pipe without low fin 13a), the evaporation heat transfer coefficient is greater by 1.08 times while the condensation heat transfer coefficient is slightly decreased to 0.98 times. If the fin height ratio becomes larger, the condensation heat transfer coefficient will be deteriorated. When the fin height ratio is 0.25, the condensation heat transfer coefficient is deteriorated to 0.8 times, and the evaporation heat transfer coefficient is greater by 1.1 times, i.e. a proportion of augmentation is low. As described above, in the conventional heat transfer pipe with grooved inner surface with the outer diameter of 7 mm having the high fins 12a with the torsion angle of 16°, the ratio of the performance improvement resulted from the addition of the low fins is low. Namely, in the heat transfer pipe 10 with grooved inner surface having the high fins 12a and the low fins 13a, the improvement in the evaporation heat transfer coefficient can be observed. However, the improvement in performance is small (less than 10%) and the condensation heat transfer coefficient is significantly reduced in accordance with the increase of the fin height ratio.
Accordingly, the Inventors of the present invention studied effect of the fin height ratio (the fin height of the low fin/the fin height of the high fin) on the ratio of the heat transfer coefficient (the evaporation heat transfer coefficient/the condensation heat transfer coefficient), and the effect of a product (P) of a inner diameter di (mm) of the pipe body, a bottom width W (mm) of a spiral groove and a sinusoidal value of a torsion angle β of the spiral groove (i.e. P=W×di×sin β) on the ratio of the evaporation heat transfer coefficient. In the process of analyzing effect of changing the fin height Hf of the high fin, the fin height hf of the low fin, the inner diameter di of the pipe body, the bottom width W (mm) of the spiral groove and the torsion angle β of the spiral groove, the Inventors found that the evaporation heat transfer coefficient can be largely improved and the reduction of the condensation heat transfer coefficient can be suppressed, when the fin height hf (mm) and the torsion angle α (°) of the low fin are determined respectively to satisfy the following conditions:Hf/15≦hf≦Hf/3 and α=β,
when P=W×di×sin β (P≧0.86) wherein the bottom width of the spiral groove is W (mm), the torsion angle of the spiral groove is β (°), and the inner diameter of the pipe body is di (mm).